Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/08—Oxides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
  • C25B1/04—Hydrogen or oxygen by electrolysis of water
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/50—Processes
  • C25B1/55—Photoelectrolysis
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
  • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
  • H01G9/20—Light-sensitive devices
  • H01G9/2027—Light-sensitive devices comprising an oxide semiconductor electrode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
  • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
  • H01G9/20—Light-sensitive devices
  • H01G9/2045—Light-sensitive devices comprising a semiconductor electrode comprising elements of the fourth group of the Periodic System (C, Si, Ge, Sn, Pb) with or without impurities, e.g. doping materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/542—Dye sensitized solar cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/10—Process efficiency
  • Y02P20/133—Renewable energy sources, e.g. sunlight
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• This invention relates to photoelectrochemical (PEC) devices for the solar photoelectrolysis of water to produce hydrogen.
• the invention piovides a photoelecirochemical (PEC) electrode or photoelectrode for use in splitting water by electrolysis.
• the photoelectrode has an electrically conductive surface in contact with an electrolyte solution.
• This surface is a doped tin oxide layer, which is in electrical contact with the semiconductor solar cell material of the PEC photoelectrode.
• Such semiconductor solar cell is preferably a triple-junction amorphous silicon (a-Si) solar cell.
• Electrolyte solutions aggressively attack many kinds of surfaces including some metals and metal oxides by corrosion and dissolution.
• the doped tin oxide layer is robust with respect to aggressive attack by the electrolyte solution.
• the doped tin oxide material is a transparent conductive oxide (TCO), thus, it is electrically conductive and transparent.
• TCO transparent conductive oxide
• Such doped tin oxide is preferably fluorine doped tin oxide (Sn0 2 :F).
• another layer of metal oxide having transparent, anti-reflective, and conductive properties is disposed between the doped tin oxide layer and the amorphous semiconductor material.
• This inner metal oxide layer may be less robust with respect to aggressive attack by the electrolytic solution than the doped tin oxide layer.
• This inner metal oxide layer is deposited on the solar cell before the doped tin oxide layer.
• Such inner layer is preferably indium-tin oxide (ITO), which is typically used as an antireflection coating.
• ITO is also a TCO material.
• the semiconductor material of the photoelec-rod ⁇ is overlaid with a transparent, anii- reflective, electrically conductive metal oxide layer and such layer is protected by being overlaid with a non-conductive, transparent material, preferably glass, polymer, or plastic which is impervious to the electrolyte solution.
• the amorphous semiconductor layer has a peripheral surface and an electrically conductive material that in one embodiment is in contact with at least a portion of the peripheral surface of the semiconductor layer and is in contact with the TCO layer.
• the electrically conductive material preferably consists of a metal, metal polymer composite, or conductive sealant.
• FIG. 1 is a schematic, sectional, representation of a photoelectrochemical (PEC) device which comprises a photoelectrode and counter electrode housed in a container with basic aqueous (alkaline electrolyte) solution; with the PEC electrode having an ITO-coated major surface facing a transparent glass shield.
• PEC photoelectrochemical
• Figure 2 is a schematic, sectional, representation of a photoelectrochemical (PEC) device which comprises a photoelectrode and counter electrode housed in a container with basic aqueous solution; with the PEC electrode having an ITO-coated major surface facing a doped tin oxide coated transparent glass shield.
• Figure 3 is a schematic, sectional, representation of a photoelectrochemical (PEC) device which comprises a photoelectrode and counter electrode housed in a container with basic aqueous solution; with the PEC electrode having an ITO-coated major surface, with a doped tin oxide coating overlying the ITO coating. Preferably such doped tin oxide coating is directly deposited on the ITO coating.
• PEC photoelectrochemical
• Figure 4 is a schematic, sectional, representation of a photoelectrochemical (PEC) device which comprises a photoelectrode and counter electrode housed in a container with basic aqueous solution; with the PEC electrode having a doped tin oxide-coated major surface.
• Figure 5 is a schematic, sectional, representation of a photoelectrochemical (PEC) device which comprises a photoelectrode and counter electrode housed in a container with basic aqueous solution; with the PEC electrode having an ITO-coated major surface and a glass substrate coated on all sides with doped tin oxide overlying the ITO coating.
• Figure 6 is similar to Figure 2 except that the electrically insulative epoxy does not overlie the conductive rneial-epoxy sealant
• a photoelectrochemical (PEC) device for electrolysis of water to produce hydrogen.
• the PEC device comprises a container housing a photoelectrochemical (PEC) electrode (photoelectrode), a counter electrode and an electrolyte solution.
• PEC photoelectrochemical
• the TCO-coated photoelectrode is the anode and produces oxygen
• the counter electrode is the cathode and produces hydrogen.
• the photoelectrode and the counter electrode are spaced apart from one another in the container, and each electrode is in contact with the electrolyte solution.
• the counter electrode comprises a metal such as Pt or Ni that is stable under the reducing conditions at the cathode and has a low overvoltage for hydrogen production.
• the electrolyte solution comprises a solvent which preferably comprises water, and a solute which preferably comprises a base.
• the electrolyte is a basic (alkaline) aqueous solution.
• Use of an acid instead of a base is also possible. An acid is not recommended due to corrosion problems, but use of an acid or neutral salt in the electrolyte solution instead of a base is within the scope of the invention.
• the photoelectrode comprises a semiconductor layer, typically and preferably triple-junction a-Si, having first and second major surfaces.
• the first major surface is an electrically conducting substrate.
• the first major surface is stainless steel (ss) on top of which is deposited a layer of silver, a layer of Zn ⁇ 2, and then three layers of n-type, i-type, and p-type semiconductor materials (see Deng and Schiff, 2003, "Amorphous Silicon Based Solar Cells," Chapter 12, pages 505-565 in Handbook of Photovoltaic Engineering, ed. A. Luque & S.
• the second major surface is a robust transparent conducting and transparent metal oxide layer that is in contact with a first metal oxide layer which comprises a first transparent, anti-reflective, and electrically conductive metal oxide material.
• a second or outer metal oxide layer comprises a second transparent and electrically conductive material. This second layer is adjacent the second major surface of the semiconductor.
• the second metal oxide layer is arranged in electrically conductive contact with the first metal oxide layer; and the second metal oxide layer is more stable in basic solution than the first metal oxide layer.
• the second layer forms the electrode surface where evolution of gaseous electrolysis product, typically oxygen, occurs.
• the first metal oxide material comprises indium tin oxide, ln 2 ⁇ 3 :Sn0 2 , referred to as ITO.
• the second metal oxide material is fluorine-doped tin oxide (SnQ 2 :F).
• the 8nQ 2 :F is arranged in electrically conductive contact with the first TCO layer and is between the first TCO layer and the electrolyte solution.
• the invention provides a photoelectrode comprising a semiconductor layer having a first major surface in contact with an electrically conductive substrate and a second major surface in contact with a transparent, electrically conductive doped tin oxide (Sn0 2 ) layer; wherein the semiconductor comprises a photovoltaic, a-Si triple junction material.
• the invention provides a photoelectrode comprising a semiconductor layer having a first major surface in contact with an electrically conductive substrate, a second major surface in contact with a transparent electrically conductive metal oxide (TCO) layer.
• TCO transparent electrically conductive metal oxide
• the semiconductor has a peripheral surface defined by a thickness and an electrically conductive material is in contact with at least a portion of the peripheral surface of the semiconductor layer and is in contact with the TCO layer.
• the peripheral surface is also referred to as the outer surface or edges.
• the electrically conductive material forms the electrode surface where evolution of gaseous electrolysis product occurs.
• the generation of hydrogen using a photoelectrochemical cell requires a photoelectrode, and at least one counter electrode to the photoelectrode. Both the photoelectrode and its counter electrode are disposed in a suitable container having an electrolyte, which provides the source of hydrogen, and suitable ionic species for facilitating the electrolysis.
• the electrochemical cell typically utilizes a metal electrode such as Pt or Ni as the counter electrode.
• the reactions at the photoanode and at the counter electrode differ from the alkaline case.
• the cathodic reaction is: 2H + + 2e ' ⁇ H 2 .
• the anodic reaction is: 2H 2 0 ⁇ 0 2 + 4H + + 4e " . Notice that the H 2 is produced at the cathode (the electrode where reduction occurs) and 0 2 at the anode (the electrode where oxidation occurs) in either acidic or basic conditions.
• a basic (alkaline) electrolyte when the semiconductor photoanode is exposed to light, electrons are excited thereby creating holes in the valence band and free electrons in a conduction band.
• the electrons produced at the photoanode are conducted through an external conductive path to the counter electrode where the electrons combine with the water molecules in the electrolyte to produce hydrogen gas and hydroxide ions.
• the electrons are provided from hydroxyl ions in the solution to fill holes created by the departure of excited electrons from the photoanode, and oxygen is evolved.
• the semiconductor utilized in the system has a voltage in the necessary range to split water (1.6 to 2.2 volts) and in the preferred embodiment herein, such a semiconductor comprises a triple-junction photovoltaic type cell formed of amorphous silicon material.
• the photo-excited electrons are accelerated toward the n-layer of the semiconductor due to the internal electric field at the p-n junction.
• the holes at the p-n junction are accelerated toward the p-layer of the semiconductor.
• electrons and holes are accelerated with sufficient energy (voltage), they can react at the cathode and anode respectively, with ions present in the aqueous solution.
• Oxygen is evolved at the photoanode and hydrogen is evolved at the counter electrode (cathode) according to the reactions previously described hereinabove with respect to the alkaline or acidic solutions.
• Conventional photovoltaic cells for the conversion of light into electricity are coated with indium-tin oxide (ITO).
• Such coating on the face of such cells is typically used as an anti-reflective coating and to collect the electric current from all parts of the cell surface, so that individual solar cells can be interconnected to form solar modules and panels. Due to their corrosion, such coatings have not heretofore been found suitable for use in the aggressive environment of an electrolysis cell. [0028] Accordingly, one of the problems faced in optimizing conventional devices is the corrosion of the ITO and subsequent destruction of the semiconductor by the electrolyte. Indium-tin oxide coatings have not yet been developed to withstand the environment at such an interface.
• the design of the present invention is based upon having the p-type layer adjacent to the ITO and on having the fluorine- doped tin oxide layer exposed to the electrolyte.
• This so-called n-i-p device provides a photoanode that can withstand the corrosive anodic production of oxygen. External connection of the anode to a metal counter electrode where hydrogen evolves completes the photoelectrolysis cell.
• the hydrogen and oxygen production reactions can be physically separated so the gases do not mix.
• the very aggressive or corrosive reaction of oxygen production is occurring at the n-i-p type electrode (anode) of the present invention, there is a strong tendency for degradation of the ITO coating on the electrode at the electrolyte interface.
• the present invention addresses this difficulty by a novel design for such electrodes.
• By the present invention it is possible to use such coated cells in an electrolysis environment because the design of the present invention provides the necessary protection for such exposed ITO-coated electrode surfaces.
• PEC photoelectrochemical
• the PEC device 10 comprises a PEC photoelectrode 12 and counter electrode 20 connected by conductive wire 9.
• a shield 11 was attached to the front of a photovoltaic amorphous silicon triple-junction photoelectrode 12 using a conductive silver epoxy sealant 13.
• This is a composite material 13 consisting of a finely divided metallic powder in a polymeric resin binder.
• Glass shield 11 is arranged to protect the outer indium tin oxide (ITO), transparent conductive oxide (TCO), surface coating 14 and the underlying amorphous silicon (a-silicon) layers 15 of the device from corrosion when immersed in basic electrolytes 16.
• the metal epoxy sealant 13 was applied beneath the protective glass shield window 11 and contacted the conductive ITO layer 14 of photoelectrode 12. Note that the ITO was applied to the photovoltaic cell as an anti-reflection coating 14 and is also used to conduct electric current from the outer (p-layer) of the triple-junction a-Si solar cell 15. Thus, when the PEC device 10 was exposed to simulated solar radiation, the metallic composite sealant 13 very effectively conducted electric current from the solar cell photoelectrode 12 to the electrolyte (for example, aqueous KOH) 16 to split water and evolve hydrogen at the counter electrode 20 and oxygen at the photoelectrode 12.
• the electrolyte for example, aqueous KOH
• An indefinitely long lifetime, greater than four days when irradiated with simulated sun light with an estimated irradiance of 120 to 140 mW/cm 2 (at which time the test was ended) was observed for the present design of PEC electrode 12 having both glass 11 and the ITO coating 14.
• Oxygen bubbles are evolved at the metal composite sealant 13 (anode) of photoelectrode 12 and hydrogen is evolved at the metal cathode 20.
• the silver composite 13 was an effective catalytic surface for electrolysis lasting an indefinite time, greater than several days. Severe gray discoloration (silver tarnish) formed on the surface of the silver composite. This suggests other alternative metals may be preferred in the composite 13.
• Nickel/resin composites, including nickel powder in epoxy and other metal ters in resin binders, are appropriate lower-cost sealants to replace silver epoxy for the purpose of this invention. A mixture of several conductive and catalytic metals in a binder could also be used.
• nickel electrodes proved to be very corrosion resistant in KOH solutions and had good catalytic properties for hydrogen evolution from water splitting.
• Nickel oxides, iron oxides, molybdenum oxides, ruthenium oxides, and other transition metal oxides, and the corresponding metallic transition elements may also be added to the composite sealant (anode) or applied to the metal cathode 20 used as the hydrogen electrode to better catalyze the electrolysis reactions.
• PEC photoelectrochemical
• a glass shield 51 was cut just large enough to cover the active face of the amorphous silicon cell electrode 52 and was coated on its outer surface 59 with a thin layer of fluorine doped tin oxide (Sn0 2 :F) 57.
• the Sn0 2 :F coated glass 51 was substituted for the plain protective glass window 11 in Figure 1.
• the Sn0 2 :F a transparent conductive oxide (TCO) with sheet resistance of 15 ohms per square cm, called TEC 15 glass, was connected to the ITO 54 coating on the PEC photoelectrode 52 by the conductive metallic composite sealant 53 which acted as a waterproof adhesive and sealed the Sn0 2 :F coated glass window 51 to the front of the PEC photoelectrode 52.
• the formerly exposed areas of the metal epoxy sealant 53 were then covered with an additional, outer layer of insulating sealant (ordinary epoxy) 60 to prevent the metal conductor sealant 53 from coming in contact with the electrolyte 16.
• the transparent conductive Sn0 2 :F coating 57 was applied on the glass window 51 thus forming the oxygen electrode for splitting water (anode).
• the Sn0 2 :F coating 57 and glass 51 on electrode 52 proved to have very good corrosion resistance and catalytic capacity when immersed in 1 M aqueous KOH with the photoelectrolysis lasting indefinitely, that is more than 31 days, when irradiated with simulated sun light with an estimated irradiance of 120 to 140 mW/cm 2 .
• a nickel metal electrode 70 was connected to the metal backing 75 of the PEC electrode 52 to act as the hydrogen producing electrode (cathode).
• the complete device (PEC 50, Figure 2) operated with a steady current density of approximately 10 mA/cm 2 and evolved bubbles of hydrogen and oxygen continuously during the 31 day
• FIG. 3 there is shown a photoelectrochemical (PEC) device 80 housed in a container 58.
• the PEC device 80 comprises PEC electrode 82 and counter electrode 70 connected by conductive wire 9.
• the system of Figure 3 comprises basic components similar to Figures 1 and 2.
• similar parts to that contained in Figure 2 are marked ⁇ irnilarily.
• Figure 3 differs from Figure 2 in that the intermediate glass substrate 51 of Figure 2 is not present in Figure 3. Therefore, in Figure 3, the protective doped tin oxide conductive coating 87 is placed directly on the transparent conductive indium tin oxide coating 84.
• Such application of coating 87 onto coating 84 may be produced by a spray pyrolysis technique.
• Spray pyrolysis techniques for preparing Sn0 2 :F coatings are known and one example is described in a paper by Arcosta et al., 1996, "About the structural, optical and electrical properties of SnO ⁇ films produced by spray pyrolysis from solutions with low and high contents of fluorine," Thin Solid Films, 288, 1-7.
• Several methods for producing Sn0 2 :F films as known in the art are described, with spray pyrolysis being the least expensive.
• a spray solution of SnGU was dissolved in ethanol together with NH F, and such solution was sprayed on substrates maintained at about 300 9 C. This temperature is considered a relative upper limit for a-Si-based PEC electrode.
• the spray pyrolysis technique may be useful in producing Sn0 2 :F coatings directly on top of the ITO.
• a PEC photoelectrode 92 of Figure 4 has doped tin oxide coating 97 applied directly on top of the a-Si semiconductor material 55, and the ITO layer is not included.
• a PEC electrode 102 of Figure 5 has doped tin oxide coating 107 applied on a transparent substrate 101 , such that the substrate 101 is coated preferably on all of its sides, thereby providing a conductive path from the ITO coating 104 to and through doped tin oxide coating 107 and to the exposed surface 109 of coating 107 in contact with the electrolyte solution 16.
• the conductive metal epoxy sealant 53 required in Figure 2, may be eliminated.
• the doped tin oxide 87 may be applied directly onto the ITO 84 ( Figure 3); directly onto a first surface of glass 51 and then a second surface of glass 51 placed adjacent the ITO 54 ( Figure 2); or the ITO may be omitted and the doped tin oxide 97 placed on the semiconductor material 55 ( Figure 4); and/or the doped tin oxide 102 may be applied to several sides of glass 101 with one doped tin oxide coated surface in contact with the ITO 104 and the other doped tin oxide surface in contact with the electrolyte 16 ( Figure 5).
• the doped tin oxide coated transparent glass used in the example of Figure 2 may be purchased from Pilkington Specialty Glass Products of Toledo, Ohio USA.
• the particular type of doped tin oxide used here was flourine doped tin oxide and is available under the specification TEC Glass TM that is a trademark of Pilkington. Various designations of the TEC GlassTM are available and the specific type used herein were Pilkington TEC 7 and Pilkington TEC 15 glass coated with flourine doped tin oxide.
• the flourine doped tin oxide is known in the art as Sn0 2 :F and Sn0 2 -F, with such expressions being used interchangeably.
• Sn0 2 :F may be directly applied to the ITO, as in Figure 3, or directly to the amorphous silicon semiconductor, as in Figure 4, using a technique such as spray pyrolysis as described above.
• Figure 6 is similar to Figure 2 except that the electrically insulative epoxy 60 does not overlie the conductive metal-epoxy sealant 53. All other part numbers in Figure 6 are according to the equivalent part in Figure 2.
• electrically conductive and non-conductive epoxy systems that can be used for applications such as the aggressive basic environment of the photoelectrochemical device of the present invention.
• One such chemically resistant epoxy system which is non- conductive is available from Epoxies, Etc., of Cranston, Rhode Island and under the description of 20-3004 HV (high viscosity), which is a two component chemically resistant epoxy system with capability of good bonding to a variety of substrates.
• the electrically conductive epoxy adhesive coating was also obtained from Epoxies, Etc. and the specific type was 40-3905 which is an electrically conductive system designed for applications requiring good adhesions to a variety of substrates such as metal, ceramic, glass, phenolics, and included a solvent free epoxy system filled with pure silver as the conductive agent.
• PEC cell comprises a photovoltaic amorphous silicon triple junction cell.
• Such an amorphous silicon-based cell comprises amorphous silicon thin-film materials deposited by a preferred rf plasma enhanced chemical vapor deposition method (PECVD), as described in Deng and Schiff, 2003, "Amorphous Silicon Based Solar Cells," Chapter 12, pages 505-565 in Handbook of Photovoltaic Engineering, ed. A. Luque & S. Hegedus, by John Wiley & Sons, Ltd., such chapter separately published on Xunming Deng's website: http://www.phvsics.utoledo.edu/ ⁇ denqx/papers/denq03a.pdf in 2002 by Deng and Schiff.
• PECVD plasma enhanced chemical vapor deposition method
• Amorphous silicon and silicon germanium materials for high efficiency triple-junction solar cells are fabricated by United Solar, ECD, Fuji, University of Neuchatel, BP Solar, Canon, University of Toledo and Sharp.
• the triple-junction amorphous silicon solar cells were purchased from the University of Toledo (Professor Xunming Deng).
• the process is conducted in an ultra-high vacuum multi- chamber arrangement, in a system isolated from the environment. Preferably two deposition chambers are used. One chamber is used for the growth of a-Si and a-SiGe materials.
• the other is used for the preparation of n-type, a-Si and p-type mieroerystalline silicon ( fC-Si) layer.
• Si ⁇ H ⁇ GeH_j and hydrogen are used for the deposition of a-Si and a-SiGe materials, respectively.
• Deposition of p-layers is accomplished using BF 3 doping, while deposition of n-layers is accomplished using PH 3 doping.
• a preferred substrate is stainless steel foil, with or without a silver-zinc oxide back reflector coating, for supporting the silicon-based layers.
• the top of the silicon based electrode is covered with a layer of ITO deposited by use of an rf sputtering chamber, using various mixtures of ln 2 0 3 and Sn0 2 having predominately ln 2 0 3 and varying amounts of Sn0 2 , for example 5%, 10% and 15% Sn0 2 .
• the thickness of the ITO coating was approximately 4200 Angstroms.
• the ITO coatings were prepared by a sputtering process as described earlier hereinabove (Proceedings of 2 nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 1998) and conducted at the University of Toledo.
• the aforesaid solar cells have three pin junctions to utilize a wide range of the solar spectrum using a technique called "spectrum splitting".
• the upper cell pin junction
• the middle cell uses the visible and some portion of the infrared region, while the bottom cell uses some of the visible more of the infrared region to generate photoelectrons.
• the three cells are arranged in series so their respective voltages are added together.
• the bottom layer meaning that layer adjacent the zinc oxide/silver/stainless steel substrate in the preferred semiconductor of the present invention, is the n-type semiconductor of the bottom cell.
• the top layer meaning the layer adjacent to the ITO, is the p-layer of the top cell.
• Dual-junction a-Si/a-SiGe cell and triple junction a Si/a-SiGe/a-SiGe cells enable a "spectrum splitting" to collect the sunlight, and this achieves higher conversion efficiencies. It is known that a-Si(1.8eV)/a-SiGe(1.6eV)/a-SiGe (1.4eV) triple- junction solar cells are among the most efficient a-Si based cells. [0053] Discussion of the design, construction, and advantages of amorphous silicon solar cells, including triple-junction amorphous silicon solar cells is contained in Deng and Schiff, 2003, "Amorphous Silicon Based Solar Cells," Chapter 12, pages 505-565 in Handbook of Photovoltaic Engineering, ed.
• the invention described here comprises a photoelectrochemical (PEC) device made from an inexpensive triple-junction amorphous silicon (a-Si) solar cell that is protected from corrosion by a durable transparent and electrically conductive material. This design results in a practical method for direct generation of hydrogen by in-situ electrolysis of water.
• a-Si cells are inexpensive compared to crystalline or polycrystalline-silicon and especially compared to highly efficient but very expensive crystalline semiconductor wafers such as GaAs, GalnP 2 , and AIGaAs.
• bases may be used besides KOH, such as Na 2 C0 3 or NaOH.
• Use of acids and neutral salts are within the scope of the invention to produce the aqueous electrolyte.

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